HD Radio is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. The advantages of digital transmission for audio include better signal quality with less noise and wider dynamic range than with existing FM and AM radio. The goal of FM HD Radio is to provide virtual-CD quality stereo audio along with a capacity for a data channel. The development of new high-quality stereo codec algorithms indicates that virtual-CD stereo quality is practical at rates below 96 kbps. In-Band On-Channel (IBOC) HD Radio systems require no new spectral allocations because each digital signal is simultaneously transmitted within the spectral mask of an existing analog signal allocation. IBOC HD Radio is designed, through power level and spectral occupancy, to be transparent to the analog radio listener. IBOC HD Radio promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
IBOC HD Radio is transmitted using a composite signal that includes a plurality of OFDM subcarriers and reference subcarriers with the broadcast channel. Coherent demodulation is used for the digital portion of an FM IBOC (In-Band On-Channel) signal in IBOC HD Radio receivers. The multiple roles of the Reference Subcarriers for acquisition, tracking, estimation of channel state information (CSI) and coherent operation have been described in U.S. Pat. No. 6,549,544, which is hereby incorporated by reference. The system described in U.S. Pat. No. 6,549,544 was designed for operation in the FM broadcast band (88-108 MHz) with fading bandwidth to accommodate receivers used in vehicles at highway speeds. The various coherent tracking parameters are estimated using filters with bandwidths that approximate the maximum expected Doppler bandwidth (roughly 13 Hz). With a fixed antenna, the pertinent tracking statistics of the input signal to the tracking algorithms are assumed to vary at a rate no greater than the Doppler bandwidth.
IBOC HD Radio receivers can be used in combination with a switch diversity antenna system. The switch diversity antenna system includes multiple antenna elements (e.g., 2 to 4) usually placed within the glass of the front or back windows of a vehicle. These elements are connected to a diversity switch module which dynamically selects one or a combination of elements to provide an RF antenna signal to the receiver. The diversity switch module also monitors a signal from the receiver to determine when to switch. A typical module's “blind switching” algorithm establishes a switching threshold based on the average intermediate frequency (IF) signal level from the receiver. When the IF signal falls below the threshold, the switch blindly selects an alternative element with the expectation of yielding a better signal. If the new signal is above a threshold, then the switch maintains the new element selection. Otherwise, the diversity switch module selects an alternative element after a minimum amount of dwell time. This process continues with the switch module continually updating its threshold(s). An example of such an antenna switch diversity system is presented in H. Lindenmeier et al., “Diversity System for Receiving Digital Terrestrial and/or Satellite Radio Signals for Motor Vehicles”, U.S. Pat. No. 6,633,258 B2, Oct. 14, 2003.
The theory behind the operation of the diversity switch algorithm is based on the different instantaneous fading conditions of the various antenna elements. Multipath fading results in the addition of multiple rays (multipaths) of the signal arriving at the receiving antenna element at different times. For example a wavelength at 100 MHz is approximately 10 feet. If two signal paths arrive at a time differential of 1 wavelength or 10 nanoseconds (10 feet propagation difference), then the signals will add in-phase. Similarly if the two rays arrive at the antenna element with a time differential of a half wavelength, then the added out-of-phase signals will cancel. This addition or cancellation is dynamic in a moving vehicle where the Doppler bandwidth is approximated by BW=fc*s/c (fc is the carrier frequency, s is the speed of the vehicle, and c is the speed of light). The Doppler bandwidth is roughly 10 Hz at typical highway speeds. Therefore the signal vector (complex version of magnitude/phase) of one antenna element can vary at a rate of approximately 10 Hz in this example. Then coherent tracking of the reference signal and channel state must accommodate a 10 Hz bandwidth to maintain coherent signal tracking.
Typical antenna elements in a vehicle can experience somewhat independent instantaneous fading conditions (depending on spacing of the elements and the directions of the multiple paths). For example one element can be in a fading null while another element is at a maximum. In a vehicle with several elements, it is likely that an antenna element will receive a sufficiently higher signal while the present element is experiencing a fade (signal cancellation). Typical elements in a multi-element FM diversity antenna system will have instantaneous fading conditions that may be somewhat correlated, but sufficiently uncorrelated to achieve the desired diversity gain to improve performance.
The coherent digital modem in an example IBOC HD Radio receiver is designed to track signal fading at vehicle speeds where Doppler bandwidth is <13 Hz. The use of switch diversity antennas in vehicle windows introduces abrupt transients in the coherent tracking of the digital signal, which degrades digital performance. The transients caused by dynamic antenna switching cannot be tracked in the previous receiver modem resulting in degraded digital coverage.
This invention provides a coherent tracking method which accommodates the switching transients in a switch diversity antenna system.